An electrical steel sheet is used or transported under a corrosive environment. For example, the electrical steel sheet is used in hot and humid regions or transported by sea. During the transportation by sea, a large amount of salt comes flying. Therefore, the electrical steel sheet is required to have rust resistance. To obtain the rust resistance, an insulating film is formed on the surface of the electrical steel sheet. An example of the insulating film is a chromite-based insulating film. Though the chromite-based insulating film exhibits good rust resistance, hexavalent chromium used as the raw material of the chromite-based insulating film is carcinogenic. Therefore, it is required to develop an insulating film that can be formed without using hexavalent chromium as a raw material.
Examples of the insulating film that can be formed without using hexavalent chromium as a raw material include a phosphate-based insulating film, a silica-based insulating film, and a zirconium-based insulating film (PATENT LITERATURES 1 to 12). However, with these insulating films, the rust resistance at the same level as that of the chromite-based insulating film cannot be obtained. Though the rust resistance is improved by thickening the insulating film, the weldability and the caulking property decrease more with a thicker insulating film.